encoding/json: clarify documention for Unmarshal into a pointer.

Fixes #18730.

Change-Id: If3ef28e62f7e449d4c8dc1dfd78f7d6f5a87ed26
Reviewed-on: https://go-review.googlesource.com/36478Reviewed-by: Russ Cox <rsc@golang.org>

src/encoding/json/decode.go

@@ -22,7 +22,8 @@ import (
 )
 
 // Unmarshal parses the JSON-encoded data and stores the result
-// in the value pointed to by v.
+// in the value pointed to by v. If v is nil or not a pointer,
+// Unmarshal returns an InvalidUnmarshalError.
 //
 // Unmarshal uses the inverse of the encodings that
 // Marshal uses, allocating maps, slices, and pointers as necessary,